Communications cables which are strung between poles or those which are buried in the ground are subjected to abuse such as, for example, attack by rodents, lightning strikes, mechanical abrasion and crushing. Cables having an outside diameter below a critical size of about 0.75 inch in diameter are more apt to be damaged by rodents than are larger cables because the animals can bite directly down on them.
It has been found that with limited exceptions, the only way to protect directly exposed cables from rodent attack is to wrap them in a metallic shield. A longitudinally applied shield, if otherwise suitable, would be economically preferable from a manufacturing standpoint. For cables above the critical size, the use of a corrugated shield having a longitudinally overlapped seam has provided sufficient protection. However, in the smaller sizes, such a shield arrangement has led to failures. Rodents have been able to encompass the cable with their teeth and pull open the seam.
Both buried and aerial cables also are damaged by lightning strikes. Thermal damage, that is burning, charring and melting of the sheath components, is caused by the heating effects of the lightning arc and a current being carried to ground by the metallic members of the core or sheath. In buried cables, a second mode of damage is mechanical, causing crushing and distortion of the sheath. This results from an explosive impact, sometimes called a steamhammer effect, which is caused by the instantaneous vaporization of water in the earth in a lightning channel to the cable.
The prior art abounds with patents relating to sheath systems for copper core cables such as one comprising an aluminum shield enclosed by a carbon steel shield with each having a longitudinal seam. This sheath system offers protection from mechanical damage, eletromagnetic interference and lightning and its cost is quite low because it is made in a single pass at high line speeds. However, the use of a shield which is made of carbon steel has occasionally led to long term failures even in cables larger than 0.75 inch. Failure may occur because the underlying steel shield may become exposed when rodents violate the jacket. Once exposed, the steel, which withstands the initial attack by rodents, corrodes readily. This renders it ineffective for general mechanical protection and for protection from any subsequent animal attack. In this regard, it should be pointed out that gophers are territorial animals which repeatedly return to areas previously occupied by them. Therefore, it is not uncommon to experience secondary attacks in the same location along a cable. These cables may fail also because of the presence of a longitudinal seam formed in a shield having mechanical properties which are not sufficient to cause the shield to be protected from rodent abuse.
Lately, lightguide fiber cables have made inroads into the communications cable market. They too are subject to rodent attack and although they do not use metallic conductors for transmission, metallic strength members are used commonly in the sheath system. Given the small, comparatively fragile nature of lightguide fiber cables relative to their capacity and their contribution to revenues, provisions necessary to protect them are easily justified. Inasmuch as lightguide fiber cables fall into a range of about 0.5 to 0.8 inch in diameter, the use of longitudinal overlapped seams has been in question. A prior art design lightguide cable sheath system which offers rodent protection comprises two helically wrapped, stainless steel shielding tapes enclosed in a plastic jacket. However, it has several shortcomings. It is very expensive to manufacture because of the complex sheathing machinery and low line speeds required to wrap the tapes helically about a core and because the taping and jacketing have to be accomplished in two separate operations. Also, the protection it offers against lightning strikes is affected adversely because of the relatively low impact resistance of its helically applied flat tapes.
A cable which provides suitable protection against rodents and lightning is disclosed and claimed in copending application Ser. No. 551,797 filed of even date herewith in the names of W. D. Bohannon, Jr. et al. In it, a core is enclosed in a shield made of a highly conductive material such as copper, for example, and in a corrugated outer shield which is a laminate comprising a corrosion-resistant metallic material to which is bonded an adhesive system. The corrosion-resistant metallic material is a stainless steel alloy having relatively high elongation and at least a predetermined chromium content. The adhesive system comprises a first adhesive material which is bonded to an outer surface of the corrosion-resistant metallic material and a second adhesive material or carrier which becomes bonded to an outer jacket as plastic material is extruded about the laminate to form the jacket. Important to the corrugating and forming of the above-described outer shield is the peel strength of the bond between the first adhesive material and the outer surface of the corrosion-resistant metallic material.
In one prior art process, the coating of a corrosion-resistant metallic material is accomplished first by preheating a strip of chromium and chromium oxide coated steel. Then a polyethylene film is brought into engagement with each side of the strip and together they are fed between two rollers to cause them to be adhered to the strip. Then the coated strip is reheated and moved through a post-heating zone. This requires a somewhat lengthy manufacturing line. In another process, a dual adhesive film is extruded onto a carbon steel base layer.
What is still needed is an economical and reliable method for laminating a corrosion-resistant metallic material and an adhesive system which is capable of bonding the metallic material to a subsequently extruded jacket. The resulting composite structure should resist degradation by requiring an attacking rodent to remove laboriously each elemental piece of jacket material from an underlying durable metallic shield.